In the direct synthesis of methylchlorosilanes by the Mueller-Rochow method from silicon and chloromethane at 250° to 300° C. using copper catalysts, disilanes are obtained as by-products. The main disilanes formed are 1,1,2,2-tetrachlorodimethyldisilane, 1,1,2-trichlorotrimethyldisilane, 1,1-dichlorotetramethyldisilane, 1,2-dichlorotetramethyldisilane, chloropentamethyldisilane, and hexamethyldisilane. Of these, 1,1,2,2-tetrachlorodimethyldisilane, 1,1,2-trichlorotrimethyldisilane, and 1,1-dichlorotetramethyldisilane can be cleaved into monosilanes by catalytic reaction with hydrogen chloride in the presence of tertiary amines or amides as described in R. Calas, J. Organometall. Chem., 225, 117, 1982. The main products formed are methyltrichlorosilane, dimethyldichlorosilane, and methyldichlorosilane.
However, 1,2-dichlorotetramethyldisilane cannot be cleaved using the above-mentioned catalyst and hydrogen chloride. JP-A 54-9228 describes the use of tetrakis(triphenylphosphine)palladium(0) as a catalyst for cleavage of 1,2-dichlorotetramethyldisilane, but space time yields of dimethylchlorosilane are very low. U.S. Pat. No. 5,502,230 or JP-A 8-81477 describes the use of a catalyst mixture of (A) palladium(0) or platinum(0) and (B) a tertiary amine, carboxylic amide, alkylurea, tertiary phosphine, phosphoric amide, quaternary ammonium halide or quaternary phosphonium halide, but space time yields of dimethylchlorosilanes are very low.
The tertiary phosphine used in Example of U.S. Pat. No. 5,502,230 is triphenylphosphine. It was recognized that when the temperature lowers below 90° C. in the presence of palladium and hydrogen chloride, triphenylphosphine forms trans-dichlorobistriphenylphosphine palladium(II), losing its catalytic activity. Then, the hydrogen chloride remaining at the end of reaction must be removed from the system before the system temperature is lowered.
However, the catalyst gradually loses its activity because complete removal of hydrogen chloride from the system is difficult. Another issue associated with the use of triphenylphosphine is that triphenyl phosphine is thermally decomposed, yielding a by-product, benzene. Benzene has a boiling point approximate to that of dimethyldichlorosilane which is also formed as a by-product separately from dimethylchlorosilane, so that separation of benzene from dimethyldichlorosilane is difficult. This requires a special separation means such as a multi-stage distillation column.